<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let arr = [1,2,3,4];

        let newArr = [];
        arr.forEach((item,index) => {
            newArr[index] = item * 10;
        })
        console.log(newArr);

        let newArr2 = arr.map(item => {
            return item * 10;
        });
        console.log(newArr2);

    </script>
</body>
</html>